The present invention relates to the field of optical tele- and data-communications networks. More particularly the present invention relates to increasing the capacity throughput or the reconfigurability of such networks.
Optical Code Division Multiple Access (CDMA) is a viable multiple access format that permits efficient network control and switching, primarily because it overcomes the limitations of switching time associated with optoelectronic devices and the processing delay associated with network protocols. These switching time and processing delay limitations are particularly serious for ultra dense, Gigabit per second (Gb/s) networks. CDMA permits combined use of wavelength, frequency, time, and space, therefore increasing the information capacity of communication systems and devices. CDMA, unfortunately, has limitations associated with the crosstalk of its codes and its bandwidth expansion requirements. These CDMA limitations have been considered serious enough to hinder CDMA from being a contender for any of the emerging ultra dense, Gb/s systems. However, it is known that the crosstalk limitation of CDMA codes can be reduced by suitable code design and signal processing, and the bandwidth requirements of CDMA can be reduced by conditioning the code on several waveform attributes rather than only a single dimension, such as time, as in optical orthogonal codes (OOC), or wavelength, as in coherent spectral CDMA.
The problem to be solved by means of the proposed invention pertains to concurrent, non-blocking communication among P ports in a P.times.P network. The application is to communication which may be bursty and/or asynchronous and where no common system clock is to be invoked. Conventional, temporal, optical CDMA approaches have demonstrated, for example, that, using on-off pulse sequences as the coding technique, a network of eight users operating at a data rate of 1 Gb/s per port would require laser transceivers which produce, detect, and process symbols with pulse widths of about 5 picoseconds (i.e., shorter pulses than for comparable time division multiple access, TDMA systems). The objective of the invention is to maintain the same multiple access capability and data rate while relaxing pulse width (bandwidth) requirements. Coherent spectral CDMA approaches, which require transform limited femtosecond pulses, have corresponding problems and complexities.
In U.S. Pat. No. 4,866,699, Brackett et al. disclose an optical telecommunications system based on coherent spectral CDMA. At each transmitter and receiver station, Fourier components of radiation pulses are independently phase modulated. For a particular pair of transmitting and receiving stations to communicate exclusively, the modulation introduced at the transmitting station must be equal and opposite to that produced at the receiving station. This creates a kind of "lock" and "key" whereby desired information is communicated and all other is rejected. The "key" and "lock" terminology is analogous to "encoding" and "decoding".
Brackett et al.'s method of optical CDMA (1) depends on very narrow pulses (picosecond in the patent; in practice, sub-picosecond or femtosecond pulses are required) and (2) has encoding and decoding which is effected transversely to the direction of light propagation. The method relies on spectral slicing of a continuous wavelength spectrum. The encoding/decoding can be effected by means of amplitude or phase encoding. The codes typically must have approximately the same number of "1s" and "0s".
Many investigators and inventors have tried to solve the limitations of CDMA. In "Optical Code-Division-Multiplexed Systems Based on Spectral Encoding of Noncoherent Sources", Journal of Lightwave Technology, vol. 13, pp. 534-545, March 1995, M. Kavehrad and D. Zaccarin discuss CDMA as frequency encoded (FE) CDMA where spectral, not time, coding is used for additional performance advantage. In the referenced case, non-coherent, broadband sources are used instead of the narrow, pulsed, coherent sources in order to create a broad spectrum for encoding, but the total number of active channels is still limited by the available number of CDMA codes.
In U.S. Pat. No. 4,906,064, Cheung discloses an all optical switch for interconnecting a plurality of input optical fibers with a plurality of output optical fibers. The basic 2.times.2 switch unit has a first and second input, a first and second output, a mode combiner, a mode toggle control and a mode separator. All wavelengths arriving via the first input have the same propagation mode. All wavelengths arriving via the second input have an orthogonal propagation mode. Wavelengths from the first and second inputs are combined in the combiner. In the toggle control, a signal converts any of the input wavelengths completely or partially to the orthogonal propagation mode. The mode separator then separates the output wavelengths by propagation mode.
In U.S. Pat. No. 4,989,199, Rzeszewski discloses an optical multiplexer and demultiplexer using combined code division and wavelength division multiplexing. The multiplexer comprises a plurality of code division multiplexers, each responsive to a plurality of input signals and a plurality of orthogonal code sequences, and a wave division multiplexer for generating an output signal representing the signals of each code division multiplexer carried on a different wavelength. Each code division multiplexer comprises a plurality of phase shifters. The demultiplexer comprises a wavelength selector, a phase shifter and a PIN diode.
The Rzeszewski patent discloses a method to provide photonic network switching by means of combining CDMA and wave division multiplexing (WDM). Rzeszewski's device receives a plurality of signals at a single wavelength at each CDMA multiplexer. The CDMA encoded signals are then superimposed (multiplexed). The superimposed output of each such CDMA block is then "color coded" by a wavelength converter. This color coding allows Rzeszewski to further superimpose the CDMA and WDM tagged signals. The decoding or "unlocking" is effected by inverting the process. This technique requires coherent optical signals for both the CDMA function (Phase shift keying, PSK) and WDM color code selection (by coherent mixing in the PIN). The CDMA codes are evidently linear (e.g., temporal pulse-position) codes. The size of the resulting network is equal to the number of CDMA codes times the number of wavelengths.
The color coding or WDM usage in the Rzeszewski patent is not special or unique because this function can equally be performed by radio frequency subcarrier modulation (RFSCM) tagging as disclosed by Mathis (see U.S. Pat. No. 4,726,644).
In U.S. Pat. No. 5,175,777, Bottle discloses switches for optical data transmission by which two or more data streams can be combined, separated or switched. Bottle discloses that, instead of buffering, as in time division multiplexing (TDK, wavelength conversions are performed.
In U.S. Pat. No. 5,245,681, Guignard et al. disclose a wavelength multiplexing device comprising a number of sources of continuous light on different wavelengths inputting light to a dendritic structure of controllable optical couplers, which are controlled by a high-speed control circuit. The multiple inputs are reduced to a single output which goes to a modulator. The device makes it possible to select at random all or part of the light sources. Thus Guignard et al. disclose a means of combining P of N wavelengths from N continuous light sources, each at a different wavelength, and modulating them with data prior to transmission via a fiber optic cable. This patent only represents a WDM array, provided that the cable is followed by a WDM demultiplexer.
In U.S. Pat. No. 4,989,937 Mahlein et al. disclose a method of manufacturing light waveguide couplers having at least three gates and a five gate wavelength multiplexer/demultiplexer operating on a beam splitter principle.
In U.S. Pat. No. 5,134,672 Imoto et al. disclose an optical waveguide star coupler including a light propagating core on a substrate. The light propagating core includes a plurality of Y-branching waveguides connected to each other.
In U.S. Pat. No. 5,179,604 Yanagawa et al. disclose a waveguide splitter/coupler including a 2-input/2-output first stage concatenated to 1-input/2-output second and subsequent stages.
In U.S. Pat. No. 5,233,453 Sivarajan et al. disclose a method and apparatus for providing high speed optical tuning. Light is routed to a tree of optical switches interconnected by waveguides. By operating the switches, the light can be directed through a desired optical filter.
In U.S. Pat. No. 5,305,412 Paoli discloses an optical array including optically amplifying waveguides coupled by passive waveguides and splitters to an input beam.
In U.S. Pat. No. 5,325,453 Drissler discloses electrical to optical converter/plugs. This invention includes light waveguides, electrical to optical transformer units and electrical connections contained in a sheath.
In U.S. Pat. No. 5,475,778 Webb discloses an optical coupler for sending and receiving optical signals over optical fibers. The optical coupler converts electrical signals to optical signals and includes circuitry for modifying the format of the electrical signals.
In U.S. Pat. No. D277,583 Kono shows a design for an optical fiber connector.
In U.S. Pat. No. 5,190,466 Kaltschmidt discloses a delay line comprising a spiral groove with taps at intervals along the spiral.
In U.S. Pat. No. 5,143,577 Haas et al. disclose a method of fabricating polymeric channel waveguides.
In U.S. Pat. No. 5,289,454 Mohapatra et al. disclose a method of reading an optical disk which includes switchable waveguides.
In U.S. Pat. No. 5,367,586 Glance et al. disclose a delay line comprising an input wavelength shifter, an input wavelength router a series of unequal length waveguides, an output wavelength router and an output wavelength shifter.
In U.S. Pat. No. 5,414,548 Tachikawa et al. disclose an optical device that can be used as a delay line. It incorporates input and output lines, feedback loops and signal processors.
In U.S. Pat. No. 5,519,803 Shiono et al. disclose an improved optical waveguide.
The paper, "Synthesis and Demonstration of High Speed, Bandwidth Efficient Optical Code Division Multiple Access (CDMA) Tested at 1 Gb/s Throughput", published September, 1994, in IEEE Photonics Technology Letters, Vol. 6, p. 1146, describes a novel method of making data streams into matrices which can be broadcast over a network and selectively received. This paper describes the theory of a class of optical CDMA matrix codes and the algorithm for deriving them from (0,1) pulse sequences. The concept can be implemented with planar technology and advanced switching techniques.
Current planar technology was illustrated by Ryo Nagase, Alira Himeno, Masayuki Okuno, Kuniharu Kato, Ken-ichi Yukimatsu, and Masao Kawachi in their article, "Silica-Based 8.times.8 Optical Matrix Switch Module with Hybrid Integrated Driving Circuits and its System Application", Journal of Lightwave Technology, vol. 12, pp. 1631-1638, September. 1994.
Some advanced switching techniques are shown by Janet Lehr Jackel, Matthew Goodman, John Gamelin, W. J. Tomlinson, Jane Baran, C. A. Brackett, Daniel J. Fritz, Robert Hobbs, Karl Kissa, Robert Ade, and David A. Smith in their article "Demonstration of Simultaneous and Independent Switching of 8 Wavelength Channels with 2 nm Spacing using a Wavelength-Dilated Acousto-Optic Switch", LEOS'95, Postdeadline Session, PD1.2, Nov. 1, 1995.
Development of optical circuitry which can greatly increase the throughput and reconfigurability of optical data networks represents a great improvement in the field of high rate data transmission and routing and satisfies a long felt need of the communication engineer.